Having conquered France, Hitler now focused his attention on Great Britain. The planned invasion of the British Isles was given the code name "Sea Lion." The German fleet commander Grand Admiral Raeder laid down an essential condition for success of the operation: the absolute superiority of the German air force. Marshal Herman Goering, commander of the German air force, was quite confident that he could chase the British out of the skies. He had at his disposal 2,800 aircraft, of which 1,400 were medium-range bombers, 300 were dive bombers, 800 were single-seat fighters (Messerschmitt 109) and 300 were two-seat fighters (Messerschmitt 110). Against this Armada, Great Britain could only muster 57 squadrons with 531 Spitfires and Hurricanes.

The Fighter Command was headed by 60-year-old air chief Marshal Sir Hugh Dowding, who would not have had much chance of winning the battle against such odds had he not possessed a great advantage, shrouded in utmost secrecy: the ability to read the most secret German radio signals, using the replica of the German coding machine "Enigma." As mentioned earlier, these codes were passed on to the British by Polish Intelligence before the war. Marshal Dowding belonged to the very small group of people who knew the Enigma secret. Since he was able to read Goering's messages, he knew his intentions and could deploy his squadrons in the most efficient manner, often incomprehensive to those who did not know the secret. In this way, he was able to compensate for the enemy's superiority in numbers.

Approximately 1,500 Royal Air Force pilots took part in the Battle of Britain, over 150 of whom were Polish. This was the largest contingent of foreign pilots fighting with the British against the Germans. Half of the Poles flew in British squadrons, and the other half in the two Polish squadrons, 302 and 303. The 302 squadron became operational on August 15; the 303 squadron followed suit on August 30, albeit in a most irregular fashion when Lieutenant Paszkiewicz shot down a German Dornier bomber without orders during a training flight. For this, he received concurrently a reprimand and a commendation. On August 2, squadron 303 was formed around the nucleus of the pilots from the old First Warsaw Air Regiment. These pilots had already bagged several kills in Poland and in France. During the Battle of Britain, squadron 303 became the highest scoring allied squadron. The Poles used their own tactics to achieve these results, and occasionally the British were skeptical as to whether the number of kills claimed by the Poles was not exaggerated.

One of the doubters was British Group Captain Vincent. On one occasion, when squadron 303 went into action, he took a plane and followed them. The squadron met a large enemy formation over the London docks. Two Hurricanes immediately climbed high above, while the rest hung back with Vincent behind them. Then, the two lone planes dived almost vertically onto the Germans, spitting fire and pretending to ram them. This forced the bombers to break formation. "The Poles behind," writes Vincent, "jumped on the scattered planes and suddenly the air was full of burning aircraft, parachutes and pieces of disintegrating wings. It was all so rapid that it was staggering." Vincent tried to join in himself, but each time he fixed on a German plane, it disintegrated before his eyes as a Pole got there first. "I returned to Northolt feeling old and musty," he writes, "and told Wilkins [the intelligence officer] that what they claimed, they did indeed get!"

The Polish successes were often ascribed by the British to "the almost incredible audacity" of the Poles. One example was the case of pilot Stanisław Skalski, who flew with the British squadron 501 during the Battle of Britain. Skalski, badly burnt the second time he was shot down, slipped out from the hospital where he was recovering and rejoined his squadron. He was left with such a terror of fire that he could not even bear to light a cigarette, but he did not admit this to his British commanding officer. He insisted upon going operational, even though his leg was so badly wounded that he could not run during a scramble and therefore had to sit in his plane for hours awaiting the signal. Eventually, he became an accomplished ace with twenty-two enemy planes destroyed, six in Poland in 1939 and sixteen in Great Britain. In 1943 he was the commander of the County of London British 601 squadron.